[llvm] [InstCombine] Canonicalize `(sitofp x)` -> `(uitofp x)` if `x >= 0` (PR #82404)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> > Regression:
> > ```
> > define float @src(i32 %shr.i) {
> >   %and.i = and i32 %shr.i, 32767
> >   %sub = uitofp i32 %and.i to float
> >   %add = fadd float %sub, -16383.0
> >   ret float %add
> > }
> > 
> > define float @tgt(i32 %shr.i) {
> >   %and.i = and i32 %shr.i, 32767
> >   %addconv = add nsw i32 %and.i, -16383
> >   %sub = sitofp i32 %addconv to float
> >   ret float %sub
> > }
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > Alive2: https://alive2.llvm.org/ce/z/W6Ik5C
> 
> Is that a regression? Seems like the `fadd` -> `add` is desirable. Likewise we don't really know return op sign in either.

https://github.com/dtcxzyw/llvm-opt-benchmark/pull/248#discussion_r1515850373
But this patch transforms `add + sitofp` into `uitofp + fadd` :(

https://github.com/llvm/llvm-project/pull/82404